Challenge

challenge.php

<?php

require_once __DIR__ . '/vendor/autoload.php';

use Symfony\Component\Process\ProcessBuilder;

// This doesn't work. The stream_select inside Process doesn't see any new
// input. We want to create a stream that can be written to asyncroniouly.
$inputStream = fopen("php://temp/", 'r+');

$processBuilder = new ProcessBuilder(array('cat'));
$processBuilder->setInput($inputStream);

$process = $processBuilder->getProcess();
$process->start();

// Doesn't have to be fwrite.
fwrite($inputStream, "We have a winner!\n\n");

fclose($inputStream);
$exitCode = $process->wait();

$output = $process->getOutput();

if (empty($output)) {
    echo "Sorry, no dice.\n\n";
} else {
    echo $output;
}

composer.json

{
  "require": {
    "php": ">=5.4.0",
    "symfony/process": "~2.0"
 }
}

It doesn't looke like Process and/or UnixPipes support proc_open() with a file resource.